Add NULLIF operator

[PJHRobles]

Is your feature request related to a problem? Please describe.

No NULLIF operator exposed for goqu

Describe the solution you'd like

Just want a helper for NULLIF similar to functions like COALESCE (goqu.COALESCE(...))

Describe alternatives you've considered

I am aware I can simply do the following:

goqu.Func(
    "NULLIF",
    table.Col("column"),
    0,
)

but it would be easier if the interface for it was exposed and added to documentation

Dialect

• postgres
• mysql
• sqlite3

Additional context
N/A

I am more than happy to implement it myself if maintainers do not have the bandwidth